Sharks are part of the Class Chondrichthyes. They are a cartilaginous fish, so their skeleton is made up completely of a tissue lighter and more flexible than bone known as cartilage. Currently, there are more than 500 known species. Sharks are a prime example of evolutionary dominance, considering they have been around for at least 200 million years. They’re also been found worldwide in various temperatures and aquatic habitats from shallow coastal waters to the open ocean. Usually a saltwater species, a few species like the bull shark are also capable of swimming in fresh, salt, and brackish water.
Sharks will have usually 5 to 7 gill slits on the side of their body near their face for breathing. Multiple rows of visible teeth line their jaws, and the teeth in the back will regularly grow in to replace old ones they lose. The smallest sharks, like the spined pygmy shark, grow to be only 7-8 inches in length, while larger sharks like whale sharks can grow to 50 feet in length. Lifespans vary between species, but many live fairly long lives anywhere from 20-30 years. Some, like whale sharks and spiny dogfish, are believed to live for more than 100 years. Sharks may swim out in the open most of the time to hunt for food, so it’s very common for them to have a darker coloration for the top half and a lighter coloration on their belly. The darker half camouflages them with the sea surface when looking down into the water, and the lighter half camouflages them with the lighter sea surface when looking above where light will be shining down. They tend to hunt primarily in the evening and at night.
Sharks are highly carnivorous and will eat anything from fish, crustaceans, mollusks, plankton, krill, marine mammals and even other sharks. They’re famous for their sense of smell that allows them to detect blood in the water several miles away. Sharks are apex predators, meaning they exist near or even at the top of the food chain in most habitats. Apex predators usually have a smaller population to start with than other species lower on the food chain. This is to balance their role of controlling the population size of species farther down the food chain. If the apex population is too vast, the lower level consumers will dwindle in population. However, researchers have found that there has actually been a massive depletion in the shark population overall, which has a cascade effect on the ocean’s ecosystems. Recovery for sharks isn’t easy because they mature very slowly, taking 12-15 years to reach reproductive age. And when they do reproduce, it’s usually only one or two pups at a time. One of the most unfortunate threats to sharks is the demand for fins in shark fin soup. Both direct harvesting of sharks and accidental catches in fishing gear have resulted in the death of an estimated 100 million sharks per year.